Is it possible to remove a copy (or several copies) of a multiple copies giveaway?

I do not know, but the best thing to do is to make a ticket and ask support.

I would think the correct topic would be one of

* Change Giveaway Game
* Delete Giveaway 
* Other

Yes it is possible but only super mods and cg can remove winners from a multi copy giveaway so expect a wait time of a few months till your problem is solved

If the giveaway still open, you can ask support to help but it better to delete the giveaway and make a new one.
If the giveaway have already end, you will need the winners consent to change the number of copies and remove their win. Best way to do this is tell them to comment on the giveaway page.
Worst case, just take a Not Received. It not that bad.

Firstly, the people that are not able to claim the key/game have to agree first to this as they can also mark the game as not received. If you can get them to agree to deletion of the key/game preferably as a comment within your giveaway, then send a ticket to support using the delete giveaway option and clarify this situation in the box provided so they know which copies to remove. As others have mentioned, only a super mod has the power to remove copies of a game of a giveaway so you will have to wait but i had my ticket resolved within a day which was superfast. Normally, it does take a bit longer. Best of luck with this as i had exactly this issue a few days ago.